Carbon Isotopic Composition of Wetlands Sediments as Indicator of Salinity and Vegetation Change in Brackish Marshes

Abstract

The stable carbon isotope r a t i o (13C/12C) of marsh sediments has been developed as a proxy for salinity change in Northem California wetlands sediments. Marshes with elevated salinity favour plant species which use the C4 photosynthetic pathway, for example, Distichlis spicata and Spartina foliosa. Many Ca plants are adapted to drought and, by analogy, to high-salinity conditions. Another important species in the saltmarshes is Salicornia virginica., which uses the CAM photosynthetic pathway. In freshwater marshes, C3 plants, such as Scirpus., Typha., Phragmites, and Juncus, are dominant. Ca plants have substantially higher 613C values ( -14%o) than C3 plants (~ -29%o), due to fractionation during carbon fixation within the plant leaf (Smith and Epstein, 1970; O'Leary, M.H., 1981). Salicornia has a 513C value of about -26%o (similar to C3). If the carbon isotopic composition of marsh vegetation is preserved in soil organic matter, the ~13C value of marsh sediments can serve as a proxy for past vegetation composition (Delaune, 1986; Chmura and Aharon, 1995). In this study, we use the 613C values of sedimentary organic carbon, in addition to the pollen composition, to evaluate changes in marsh vegetation, and hence salinity, over time. Collection of modem sediments from marshes in San Francisco Bay along a salinity gradient allowed the calibration of ~ 13C in the sediments and salinity. We further showed that the ~ 13C values of Salicoria, a CAM plant, vary with salinity between about -24%o and -28%o (with higher values occuring at higher salinity). Sediments were cored from Rush Ranch Marsh in the northern part of the San Francisco Estuary to reconstruct changes in vegetation and environmental conditions during the late Holocene. The Rush Ranch carbon isotope record shows an increase in 613C of 11%o between 45 cm and the top of the core (Fig. 1). This indicates a shift towards more salt-tolerant C4 species during the last ~ 100 years. The pollen record from the same core also indicates increased salinity during this time period. The most likely cause of the salinity change has been upstream storage and water diversion within the Sacramento-San Joaquin watershed. However reduced rainfall and freshwater inflow to the estuary was also a factor.